1. Field
This invention relates to system for indicating proximity of an Own station to any Other station that is equipped with a standard ATCRBS transponder. More specifically, the invention pertains to threat warning and collision avoidance systems for aircraft.
2. Prior Art
Many collision avoidance systems (CAS) using airborne transponder signals have been proposed. Some of these have been built and tested with varying degrees of success. The simplest systems merely receive Others' transponder replies, relying on the received signal strength to determine approximate range from Own. Other systems depend on two-way transmissions between Own and Other's transponder to provide range information. Said systems are subject to unacceptably high false alarm rates, particularly in dense aircraft environments, where reliable warnings are most needed. Still other systems involve airborne radio direction finding from Own to Other's transponders. Suitable direction finders have not been implemented, and are probably infeasible in the present state of the art.
Other systems, using the time delay between reception at 0wn's station of an interrogation message from a particular SSR and reception of an Other reply to said interrogation, are described in numerous patents, for example the following: U.S. Pat. Nos. 3,626,411, 3,858,211, 3,735,408, 3,875,570, 3,757,324, 4,021,802, 3,858,210.
These systems discriminate strongly against false alarms and provide various information about possible threats, such as range, bearing, differential altitude and identity. In general, the complexity of such a system is directly related to its capability.
There is a current and compelling need for collision avoidance systems suitable for light aircraft such as those used in general aviation. Such systems have been designated TCAS-1. They must provide basically adequate collision threat warnings, but must also be simple and economically feasible for the owners of light aircraft. A paramount requirement is that they must minimize false alarms.